1. Field of the Invention
An embodiment of the present invention relates to a semiconductor device including a transistor or a semiconductor device including a circuit which is formed by using a transistor. For example, an embodiment of the present invention relates to a semiconductor device including a transistor in which a channel formation region is formed using an oxide semiconductor or a semiconductor device including a circuit which is formed by using such a transistor.
2. Description of the Related Art
A technique for manufacturing a transistor or the like by using an oxide semiconductor film for a channel formation region and applying it to a display device has been attracting attention. Examples of such a transistor include a transistor in which zinc oxide (ZnO) is used as an oxide semiconductor film and a transistor in which InGaO3(ZnO), is used as an oxide semiconductor film. A technique for manufacturing such a transistor including an oxide semiconductor film over a light-transmitting substrate and applying it to a switching element or the like of an image display device is disclosed in Patent Documents 1 and 2.
An inverted staggered (bottom gate) transistor in which an oxide semiconductor film containing In, Ga, and Zn is used as a semiconductor layer and buffer layers formed of a metal oxide are provided between the semiconductor layer and source and drain electrode layers is disclosed in Patent Document 3. In this transistor, an ohmic contact is formed by intentionally providing the metal oxide layers as the buffer layers between the semiconductor layer and the source and drain electrode layers.